Anti-skid system



June 17, 1969 L. BALLARD I SKID SYSTEM Sheet 1 of 2 Filed Feb. 9, 1968 mQE 8 5 5K v mw s34 EB q Q a .m wwri mwI. .O EOE m zzgm P312- INVENTOR.

ROBERT L. BALLARD ATTORNEY.

June 17, 1969 R. L. BALLA'RD 'ANTI-SKID SYSTEM Sheet Fil ed Feb. 9,1968- ATTORNEY United States Patent O US. Cl. 303-21 6 Claims ABSTRACTOF THE DISCLOSURE A plurality of electromagnetic sensing devices arerespectively associated with the wheels of a vehicle. Each deviceproduces a signal corresponding to the speed of rotation of itsassociated wheel. The signals from the wheels are individually comparedwith the signal derived from the fastest turning wheel to producerespective error signals which are applied to respective pressurerelease valves in the hydraulic brake lines for the slower turningwheels. Each error signal causes the intermittent release of thepressure on the brake to which it is applied for time intervals whoseduration is directly proportional to the magnitude of the error signal.

BACKGROUND OF THE INVENTION There is a demonstrated need for systemswhich tend to prevent or diminish skidding of vehicles on ice, wetroads, and the like which occurs when the vehicle operator applies thebrakes. The skidding action, which the present system is designed tocounteract, results in rotation of the wheels at different velocitiesdespite the application of nominally the same braking pressure to allwheels.

BRIEF STATEMENT OF THE INVENTION I have found that in the type ofskidding just mentioned, the skidding may be halted or diminished byadjusting the brake pressure applied to all wheels. In accordance withthe invention, a system is provided for detecting the speed of eachwheel considering the fastest-turning wheel as the referencenon-skidding wheel then reducing intermittently the braking pressure onall Wheels turning significantly slower than the fastest wheel forrespective time intervals which are directly proportional to thedifierences between the velocity of the fastest wheel and the velocityof each of the slower turning wheels.

DESCRIPTION OF THE DRAWINGS FIG. 1 is a block and schematic diagram ofone form of the invention.

FIG. 2 is a schematic circuit diagram of one of the components shown inFIG. 1.

FIG. 3 is a graph depicting the brake pressure release action of thesystem shown in FIG. 1.

FIGS. 4 and 5 are enlarged sectional views of part of the apparatusshown in FIG. 1 in two different operating conditions.

DETAILED DESCRIPTION OF THE SYSTEM FIGURE 1 shows one part of theoverall system constructed in accordance with one form of the presentinvention. Apart from the comparator 19, FIG. 1 illustrates theapparatus for one wheel only, although it should be understood thatthere would be three other similar arrangements for three other wheelsof the vehicle (in the case of a four-wheel vehicle). There is shown thebrake drum 5 of one wheel of a vehicle. Also shown are the spring-biasedcaliper brake shoes 6 which are coupled to the wheel cylinder 8. Thewheel cylinder is actuated by the application of hydraulic pressure fromthe master cylinder 4 through the valve 21 and through hydraulic line 20when the brake pedal 3 is pressed.

One embodiment of this invention uses disc 11 of magnetic propertiesattached to the axle of the wheel, having one or more magneticdiscontinuities, such as an aperture 12 formed therein. Positioned closeto the disc 11 is a permanent magnet 13 around which are wound a numberof turns of wire 14 whose ends are connected to the input of a pulseamplifier 15. It will be seen that as the wheel and disc 11 revolve,each time the aperture 12 passes one pole piece of the magnet 13, theflux distribution and intensity of the magnet will change therebyinducing a voltage pulse in the turns 14. This pulse is applied to theamplifier 15 which amplifies it to an amplitude sufiicient to initiatethe cycle of the one shot multi-vibrator 16. When the amplified pulse isapplied to the input of the multi-vibrator, the latter producesalternate positive and negative half cycles. The amplitude and width ofthe halfcycles are determined by the parameters and values of themulti-vibrator circuit.

The output signal from the multi-vibrator 15 is applied to a rectifieror clipper 17 which extracts only the negative half cycles and appliesthem to a low pass filter 18. When a train of such negative pulses isapplied to this filter, it produces a negative DC output voltage wavewhich is really an analog signal whose amplitude is directlyproportional to the speed of rotation of the associated wheel. Thisanalog signal is applied to one input of a signal comparator 19 whoseoperation will 'be explained in detail in connection with FIGURE 2below. It should be noted that the components shown in FIG. 1 ahead ofthe comparator are used to derive the velocity analog voltage and can bereplaced by equivalent means for producing a tachometer signal.

There are three other inputs to the signal comparator 19 from the threeother wheels of the vehicle and their associated apparatus. Thecomparator 19 is so constructed that it will compare the signalrepresenting the velocity of the fastest wheel (which is considered tobe non-skidding, or skidding less than some other wheels) individuallywith the signal from each other wheel. It will thereupon produce errorsignals for the wheels which indicate the respective differences betweenthe velocity of the fastest wheel and that of each of the other wheels.The comparator also includes a delay (RC) circuit which sets the minimumthreshold of corrective sensitivity. This delay is introduced to permitthe system to be insensitive to minor diflerences in wheel speeds thatarise when the vehicle turns a corner, when the tire diameters areunequal, etc. The comparator also produces the error signal pulses at anunvarying repetitive rate but the pulses themselves have a width whichvaries directly with the difference between the velocity of the fastestwheel and the particular wheel involved.

These error signal pulses are applied to a solenoidactuated pressurerelief valve 21 which is coupled to the hydraulic lines 9, 10, and 20.Line 10 returns to the fluid reservoir of the master cylinder 4. On eachactuation of the valve 21, the armature associated with the solenoid 21ais retracted, thereby allowing the hydraulic pressure in line 20, in thewheel cylinder 8 and on the brake shoe to be at least partly reduced bythe diversion of some of the fluid back to the reservoir via line 10(FIG. 5). At the same time, line 9 is blocked o-if by the valve (FIG. 5)to prevent any efiect on the other hydraulic lines going from mastercylinder 4 to the other brakes. As a result, the slipping wheel willtend to speed up to match the speed of the faster turning Wheels.

If it is assumed that the wheel with which the apparatus shown in FIG. 1is associated has the lowest rotational velocity of all four wheels, itsvalve 21 will be actuated by pulses applied thereto whose widths aregreater than the width of the error signal pulses applied to thecorresponding valves associated with the faster wheels. Consequently,over a period of time, the slowest wheel will have the least total brakepressure applied to it and the intermediate speed wheels will havesomewhat more total brake pressure applied to them since their reliefpressure pulses will be of shorter duration. The brake pressure appliedto the fastest wheel will not be reduced. As a result, the rotationalvelocities of the wheels other than the fastest wheels will all tend tocatch up in speed and thereby diminish or overcome the skidding.

FIGURE 2DETAILED EXPLANATION Referring now to FIGURE 2 there are shown aplurality of gas tubes 25, 26, 27 and 28 having their respective anodesconnected through a relay coil 31 to an appropriate source 59 of apositive-polarity sawtooth wave. Connected to the cathodes of each ofthese tubes are load or limiting resistors 29, 30, 45 and 32,respectively. The various negative input signals applied to the cathodescome from the low pass filter 18 and its counterparts in the systems(not shown) for the three other wheels. Also connected to the sawtoothvoltage source 59 is contact 5112 and one plate of a large valuecapacitor 52 whose other plate is connected to ground.

As stated above, there will be four negative input signals to thecathodes of the tubes 25-28. To simplify the explanation, the operationof the circuit will be explained on the assumption that thefastest-turning wheel is connected to the tube 25 and its associatedtriode 35. The operation of the other diode-triode pairs and theirassociated circuits is, of course, the same. A positive sawtooth voltagewave, at a frequency of say cycles per second is initially applied tothe anodes of all the gas tubes through relay coil 31. Assuming that theignition potential for the gas diodes is 95 volts and the sawtoothvoltage attains +50 v. DC during its rise time, when the analog voltagefrom filter 18, which may be for example, 45 volts, is applied to thecathode of gas tube 25, it will fire diode and current will flow throughit and the load resistor 29 to ground. As soon as gas tube begins toconduct, the relay coil 31 will be energized. Current through the relaycoil 31 shifts the arms of the associated cathode relay switches fromcontact with relay contacts 33b, 36b, 39b, and 4222, into contact withcontacts 33a, 36a, 39a, and 42a respectively. As a result, since theanalog signal which had been applied to the cathode of tube 25 wassimultaneously being applied to the capacitor 34, the stored voltage incapacitor 34 as it discharges through resistor 53 to ground will cause acorresponding negative voltage to be applied to the grid of tube 35. Theanalog signals which were being applied to the other gas diodes willlikewise be stored in respective capacitors 37, 40 and 43 and they willalso be applied to their associated triodes.

It should be noted that while the sawtooth voltage was being applied tothe anodes of the diodes, it was also being used to charge up capacitor52. The voltage in the capacitor 52 will be a function of the time inthe sawtooth cycle at which the first gas diode conducts. Hence, whenthe solenoid 31 is energized and the relay arm 57 will move to touchcontact 51a, the +50 v. DC stored in the capacitor 52 will thereby beapplied to the anodes of the tubes 35, 38, 41 and 44. In other words,the stored DC voltage in capacitor 52 becomes the B\- voltage for thesetriodes for each sawtooth cycle. The -45 volt analog signal stored incapacitor 34 will be applied to the grid of tube and will keep thetriode 35 cut off. Consequently, no current will flow through thesolenoid 21a to release the brake pressure against wheel No. 1 thefastest turning (the non-skidding) wheel. It has thus been seen thatsince the fastest turning wheel will generate the most negative analogsignal, it will, by firing gas tube 25 sooner than any other diode,cause coil 31 to be energized thereby preventing any of the other analogsignals from firing the other gas diodes during the same cycle ofsawtooth voltage. Thus, the fastest turning wheel determines at whatpoint on the rise time of the sawtooth wave the coil will be energizedand hence determines the amplitude of the voltage applied to the anodesof the triodes for comparison with the analog voltages respectivelyapplied to the triode grids from their associated capacitors.

Although the analog signal from the fastest-turning wheel actuatessolenoid 31, it is so negative that it keeps its associated triode cutoff. On the other hand, the less negative analog signal from a slowerwheel, say wheel No. 2, which was stored in the capacitor 37 just beforeits relay arm switched into contact with contact 36a, is applied to thegrid of triode 38. The passage of current through resistor 58 and triode38 will cause a voltage drop so that the voltage on all of the otherplates is reduced somewhat. Since this signal is less negative than thatapplied to the grid of the tube 35, the triode 38 will begin to conductif the grid voltage is insutfciently negative relative to the reducedplate voltage to prevent the flow of current through the triode. It willcease cOn duction when the grid-plate voltage-relation is in suflicientto enable conduction. Conduction of the triode 38 causes therelief-valve solenoid 46 to be energized so that the brake pressure on aslower wheel No. 2 is relieved. If the stored analog signals applied tothe grids of the other triodes 41 and 44 at the moment when coil 31 isenergized, are more negative than the signal applied to the grid oftriode 38 (but are not as negative as the voltage applied to triode 35),they will also cause their associated triodes to conduct for intervalsof time which are shorter than the conduction intervals of the triode38, i.e., which are inversely proportional to the magnitude of theirrespective analog voltages. If there is a wide variation between thevelocities of a badly skidding or lockedup wheel and some other wheelwhich is skidding only slightly, the triode associated with that otherwheel may not conduct at all. This has the efiect of relieving thepressure on the wheels in need of relief, while providing a plateau ofminimum skid under which no corrective action will take place. This willalso take care of normal minimal variations in the speeds of the fourwheels attributable to tires having different tread depth or inflation,turning of corners, etc.

FIGURE 3 illustrates how the brake pressure applied to a particularwheel, say wheel No. 2 in the example explained above, is periodicallyrelieved for varying intervals of time. It sets this out graphicallyagainst the occurrence of the sawtooth voltage from generator 59, thesawtooth being shown in a broken line. During the time interval from Tto T the wheel is starting to skid, i.e., to rotate more slowly than oneor more of the other wheels. The pressure release interval A shows thebeginning of the corrective action produced b the diode 26-triode 38combination. The width of the interval A is determined by how long ittakes for the analog voltage stored in capacitor 37 to dischorge throughresistor 54 with a value such that the grid of the triode 38 is notnegative enough relative to the plate voltage to prevent conduction. Asthe skid develops on wheel No. 2 during the interval T -T it tends torotate even more slowly (i.e., to skid more), so its analog signalapplied to the grid of triode 38 will be even less negative and thetriode will begin to conduct even earlier in the sawtooth cycle. Thenext pressure relief interval B is therefore longer than interval A. Thelongest interval C during T -T shows the maximum corrective action asthe wheel speed decreases even more. However, once the corrective actiondue to the diminution of the brake pressure during these intervals hastaken effect, the wheel No. 2 will begin to pick up speed during T -TConsequently, it will generate a more negative analog voltage, therebyshortening the relief interval D. Similarly, intervals E and F shows theprogressive reduction of the length of the '5 relief intervals as thewheel speed begins to increase, thereby approaching that of the fastestturning wheel.

GENERAL REMARKS It is evident that other ways of producing the analogvoltage from each wheel are also possible. For example, a permanentmagnet could be mounted on the edge of a disc which rotates with thewheel. The disc would pass through a winding thereby inducing a pulsetherein for each revolution. If a DC tachometer is substituted forcomponents 12 through 18 the analog voltage will be directly derived butthis may be a relatively costly alternative. This alternative also hasthe disadvantage of employing many more moving parts than thepredominantly electrical form shown in FIG. 1.

It should be borne in mind that this system is not intended to overcomeall types of skidding. For example, it will not be effective whenskidding occurs during the time that all four wheels are renderedimmobile by the brakes. Nor will it be effective in the case of strictlysideways skidding. The present system is not sensitive to yaw butessentially only to forward motion of the wheels.

Also, the gas diodes and the triodes can naturally be supplanted bytheir solid state equivalents.

What I claim is:

1. A system for equalizing the rotational velocity of a plurality ofrotating members each of which is associated with a braking means,comprising:

(a) means for sensing differences in the rotational velocities ofselected ones of said members with respect to the rotational velocity ofthe fastest rotating one of said members, and

(b) means responsive to said differences for adjusting at an unvaryingrepetition rate, the braking action exerted by the respective brakingmeans associated with said selected members, said adjustment being anintermittent reduction in the braking action exerted by said respectivemeans on their associated members for varying intervals of time, thelength of said intervals being a direct function of the magnitude ofsaid sensed dilferences.

2. The system according to claim 1 wherein said (a) means includes (1) aplurality of means for generating a plurality of signals respectivelycorresponding to the rotation of each of said plurality of rotatingmembers, each of said signals comprising a train of constant amplitude,constant width rectangular electrical pulses,

(2) a plurality of means to which said plurality of pulses arerespectively applied for producing respective corresponding analogsignals whose amplitudes are directly proportional to the speeds oftheir associated rotating members, and

(3) signal comparison means including'a source of a sawtooth wave towhich said analog signals are applied for producing error signalsrespectively indicative of the difference between the velocity of thefastest rotating member and the other rotating members.

3. The system according to claim 1 wherein said rotating members are theWheels of a vehicle and said (a) means includes:

means respectively associated with each of said wheels for producing ananalog signal indicative of the rotational velocity of said wheel whenits associated braking means is applied thereto and further wherein said(b) means includes:

(i) means for comparing the analog signal from the wheel rotating at thehighest velocity with the respective analog signals from the otherwheels thereby to derive a plurality of error signals, and

(ii) means coupled to each of said braking means for intermittentlyreducing the pressure thereof against its associated wheel in responseto a respective one of said error signals, the duration of the intervalsduring which said intermittent reductions occur being a direct functionof the amplitude of said one error signal.

4. The system according to claim 3 wherein: said (a) means includes:

(i) magnetic rotating members attached to or part of each of saidwheels, each member including a discontinuity formed therein,

(ii) a plurality of magnets respectively mounted in proximity to saidmembers,

(iii) a plurality of turns of a conductor wound around each of saidmagnets and in which voltage pulses are induced when saiddiscontinuities pass said magnets, I

(iv) means responsive to said voltage pulses for 20 producing electricalanalog signals and wherein:

said (b) means includes signal comparator means to which said analogsignals are applied, said comparator means including:

(i) a plurality of switching conduction devices to which said analogsignals are respectively applied,

(ii) means for applying a sawtooth wave to all of said (i) devices,

(iii) a plurality of variable conduction devices,

(iv) a plurality of switching means respectively connected between oneof said switching conduction devices and one of said variable conductiondevices,

(v) a plurality of capacitive means respectively connected to difierentones of said (iv) switching means,

(vi) a signal storage means switchably connected to saidsawtooth-applying means and to said plurality of switching means, and

wherein said (b) means further includes a plurality of brake pressurerelief valves connected to respective ones of said conduction devicesfor actuation thereby when energized by said conduction devices therebyto reduce the pressure of the braking means associated therewith.

5. The system according to claim 4 wherein said means for producing saidelectrical analog signals includes a one-shot multivibrator whichproduces a plurality of positive half-cycle rectangular pulses followedimmediately by respective negative half-cycle rectangular pulses, arectifier for extracting substantially only said negative half-cyclepulses, and a low pass filter to which said extracted negativehalf-cycles are applied which produces a negative DC voltage in responsethereto which is an analog of the speed of rotation of the associatedwheel.

6. The system according to claim 4 wherein said switching conductiondevices are gas tubes and said variable conduction devices are tubeshaving at least three electrodes.

References Cited UNITED STATES PATENTS 4/1966 Thompson et al. 303-217/1966 Packer 303-21 MILTON BUCHLER Primary Examiner.

I. I. MCLAUGHLIN, JR., Assistant Examiner.

US. Cl. X.R.

